[clang] [UBSAN] add null and alignment checks for aggregates (PR #175032)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> @vasu-the-sharma, can you move this into draft state while we get #175478 and #175023 into shape?

@vasu-the-sharma, I asked you to move this into draft state. At this point, I think you might want to close this PR and hold off until an internal review of this last PR in the stack determines that it is upstream-ready.

https://github.com/llvm/llvm-project/pull/175032